Strava depends a bit. I don‘t know whether your GPS device has a barometric altitude sensor. If it doesn‘t have one, it will determine altitude by GPS which has a few meters error there, too, and therefore produce data that are particularly bad for Tacx GPS Rides. Strava will replace any uploaded non-barometric altitude data with their own digital elevation model which is notorious, but they also gradually introduce an elevation model based on averaged data from the gazillion of rides uploaded from GPS devices having barometric altitude sensors. So you can get bad or perfect data from them depending on how and where they were generated and processed.

Strava depends a bit. I don‘t know whether your GPS device has a barometric altitude sensor. If it doesn‘t have one, it will determine altitude by GPS which has a few meters error there, too, and therefore produce data that are particularly bad for Tacx GPS Rides. Strava will replace any uploaded non-barometric altitude data with their own digital elevation model which is notorious, but they also gradually introduce an elevation model based on averaged data from the gazillion of rides uploaded from GPS devices having barometric altitude sensors. So you can get bad or perfect data from them depending on how and where they were generated and processed.

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It's a Garmin with both barometric and GPS. I use Altitude and Distance Correction (by Google) in Strava. Will try the corrected GPXs today-tomorrow

I don‘t know whether your GPS device has a barometric altitude sensor. If it doesn‘t have one, it will determine altitude by GPS which has a few meters error there, too, and therefore produce data that are particularly bad for Tacx GPS Rides. Strava will replace any uploaded non-barometric altitude data with their own digital elevation model which is notorious, but they also gradually introduce an elevation model based on averaged data from the gazillion of rides uploaded from GPS devices having barometric altitude sensors. So you can get bad or perfect data from them depending on how and where they were generated and processed.

The GPS files that I generated from my personal real life rides all derived their altitude data barometrically. I found the barometric altitude data were pretty good for ride purposes. So, even though barometric pressure may drift during a long day and have a minor affect on altitude readings, the changes were not sudden. But my home made GPS still had lots of glitches that required editing and smoothing in order to work well. There was still jumps and disconnects and other discontinuities caused by loss of GPS signal and waiting at traffic lights.

You can often find good GPS courses on bike ride websites. As I've stated elsewhere, the GPS function is an interesting novelty and kind of fun to play with initially, but after a while it just seemed I lost interest in using the GPS function in TTS with most indoor riding spent with nice videos and only very occasional VR courses.

It's a Garmin with both barometric and GPS. I use Altitude and Distance Correction (by Google) in Strava. Will try the corrected GPXs today-tomorrow

Altitude correction need not, but may be the main issue. This replaces good barometric altitude data with bad digital elevation model data unless you hit those roads that have that new averaged barometric elevation data.

As I've stated elsewhere, the GPS function is an interesting novelty and kind of fun to play with initially, but after a while it just seemed I lost interest in using the GPS function in TTS with most indoor riding spent with nice videos and only very occasional VR courses.

I agree. GPS Rides may help systematic training if you ride the same route outdoors and indoors year-round, but personally I very much prefer Real Life Videos.